In an Internet Protocol version 4 (IPv4) network, a source node can send a datagram of any size that its local link can handle. At least one router along the datagram's path may fragment the datagram, if such fragmentation is needed. However, this approach to datagram fragmentation increases the load on the network's routers and causes degradation in their performance. Since it is much more efficient for a router to forward datagrams intact rather than to occupy its processor with the task of datagram fragmentation, delegating the responsibility of adjusting the datagram to an appropriate size from the routers to the source node is a called for approach. Indeed, the Internet Protocol version 6 (IPv6) standards adopt the latter approach and specify that fragmentation of packets (datagrams) has to be performed only at the source node (as stated in RFC 2460, paragraph 4.5: “unlike IPv4, fragmentation in IPv6 is performed only by source nodes, not by routers along a packet's delivery path”). Thus, in an IPv6 network, when a packet arrives at an intermediate link associated with a Maximum Transmission Unit (MTU) size smaller than the packet size, the packet is discarded and an Internet Control Message Protocol Packet Too Big (ICMP PTB) message is sent back to the packet's origin (as defined in RFC-1981).
In general, the term maximum transmission unit may refer to any characteristic that may be associated with a link and which may limit the transmission of a packet over that link due to the packet having a property exceeding and/or conflicting with said characteristic of that link, including, for example, a Maximum Transmission Unit (MTU) characteristic under IPv6 and/or a similar characteristic under another protocol other than IPv6. Similarly, the packet too big indication may be any indication that a packet exceeds some predetermined size and may include, for example, a PTB message under IPv6 and/or a similar indication in another protocol other than IPv6.
An IPv6 network can include a high-latency link followed by several other links. In such a network, it is possible that each one of the other links will be associated with a different MTU. In the worst case scenario, these other links are arranged in a descending MTU order. If the source node has no knowledge of the smallest MTU along the path from the source node to the final destination of the packets being transmitted, packets may be discarded by one or more routers controlling the said other links until the ICMP PTB message corresponding to the link having the smallest MTU arrives at the source node and the source node accordingly adjusts the size of the packets it sends to the destination.
Let TI be the round trip time measured from the time a packet is sent by a source node until an ICMP PTB message is received at the source node.
Let BR be the transmission rate at the source node.
Let n be the number of links in the path (i.e., between the source node and the destination of the packets being sent by the source node) with MTU smaller than the packet size originally used by source node, and assume the links in the path after the high-latency link are arranged in descending MTU order.
Thus, the number of Bytes discarded during a TI time interval equals the transmission rate times TI (Discarded Bytes=BR*TI) and the total latency in delivering a packet (and hence, of an entire session) will be n*TI. In a satellite network, TI is relatively long, typically longer than 500 milliseconds, wherein the terrestrial network after the satellite link prolongs the above interval even further.
In order to overcome the delay of high-latency links, acceleration techniques are applied, such as Transmission Control Protocol (TCP) spoofing. In a TCP spoofing technique, a node at the near edge of the long latency link simulates a destination node located at the other side of the long latency link in order to enhance TCP performance. This edge node then uses a protocol better suited for communicating over the long latency link in order to reliably convey the information to the destination node at the other side of the long latency link.
Significant packet loss that could be caused by MTU mismatch along the packet transmission path may interfere with TCP acceleration techniques. Such interference may result in detrimental effects such as additional delay (including TCP back-off delay), throughput reduction (due to TCP mechanisms detecting packet loss and reducing the transmission window size), bandwidth waste (due to retransmission of packets) and unpredictable results when an ICMP PTB is received at the source node for a packet that was already acknowledged by the TCP spoofing mechanism.